1. Field of the Invention
The present invention generally relates to a technology for optical transmission, and particularly relates to a system for detecting a fault in an optical transmission path connecting an optical-communications apparatus and an optical amplifier corresponding to the communications apparatus.
2. Description of the Related Art
In an optical-transmission path, and more specifically in communications using optical fibers, unrepeatered systems may be adopted that have along the way no repeaters requiring feeding. In such unrepeatered systems, as a method of increasing the transmission distance, there is a method of placing an optical amplifier having an Erbium-Doped Fiber (Below represented as “EDF”) along the transmission path so as to excite this EDF by a light beam transmitted from a terminal station (a communications apparatus at the transmitter or a communications apparatus at the receiver). When trying to increase the transmission distance, a light beam is sent from the terminal station to the EDF within the optical amplifier by means of an optical fiber dedicated to an excitation light-beam that is different from an optical fiber for a signal light-beam (For example, Non-patent document 1: P. B. Hansen, L. Eskilden, S. G. Grubb, A. M. Vengsarkar, S. K. Korotky, T. A. Strasser, J. E. J. Alphonsus, J. J. Veselka, D. J. DiGiovanni, D. W. Peckham, E. C. Beck, D. Truxal, W. Y. Cheung, S. G. Kosinski, D. Gasper, P. F. Wysocki, V. L. da Silva and J. R. Simpson, “529 km Unrepeatered transmission at 2.488 Gbits/s using dispersion compensation, forward error correction, and remote post and pre-amplifiers pumped by diode pumped Raman lasers”, Electronic Letters, vol. 31, 1995, pages 1460-1461, and http://www.alcatel.com/submarine/products/ur/).
In such unrepeatered systems, the outgoing power levels of the signal light-beam and of the excitation light-beam at the transmitting communications apparatus respectively may even reach a level of 1 W or more. Therefore, when an optical fiber has been cut by chance, a light beam irradiated from the cut face may be hazardous to the human body. Moreover, there is a danger that a phenomenon called fiber fusing in which an optical fiber burns may occur. Therefore, from a point of view of achieving safety, there are methods that detect, when an optical fiber connecting a terminal station that is often constructed onshore and an optical amplifier has been cut, a reflecting light-beam generated at the cut face, and that stop the light beam from being output from the terminal station (refer, for instance, to Patent Document 1: JP08-29835A); and that have both terminal stations send to each other a supervisory-control signal of a low outgoing power level before sending out a signal light-beam and an excitation light-beam so as to send the signal light-beam and the excitation light-beam after confirming continuity (refer to, for instance, Non-patent document 2: T. Otani, T. Maki, H. Deguchi, H. Irie, T. Takahashi, E. Ishikawa, D. Ikeda, S. Harasawa, “10G×32-wave 250 km unrepeatered transmission system development”, the Institute of Electronics, Information, and Communication Engineers, Lecture Notes of the 2003 General Conference, page 474, B.10.44).
However, with the method of detecting the reflecting light-beam generated at the cut face of the optical fiber and stopping the light beam from being output from the terminal station, the reflecting light may not be detected due to the reflection being small depending on the shape of the cut face. Moreover, when the cut face is immersed in a liquid such as water having a small refractive index relative to glass, etc., the reflecting light may not be detected with the reflection being so small.
Furthermore, in the method of having both terminal stations send to each other the supervisory-control signal of the low outgoing power level before sending out the signal light-beam and the excitation light-beam so as to send the signal light-beam and the excitation light-beam after confirming continuity, when the distance between the terminal stations is long, there may be a case such that as the outgoing power level of the supervisory-control signal sent from one terminal station is low, the other terminal station is not able to receive this supervisory-control signal due to attenuation in transmission.